I think it depends on the dealership, when I ordered my XC40 they required £1000 deposit even though I used a grant, I said no, they then said £500 I again said no, I told them the grant of £1695 (I think) should be enough. They said the extra I paid would be refunded back to me on pick up of the vehicle. I refused.
in the end they gave in and with the dealership discount I ended up only paying £54 which I paid to keep them happy.
i think I have only paid a deposit on 2 cars, the GTD £150 and xc60 £500 I think.